1. Field of the Invention
The present invention relates to a method for controlling the thickness of rolled strips and for compensating irregularities caused by roll eccentricities, in which the size of the roll gap, the rolling force and the stand modulus are taken into consideration.
The present invention also relates to an arrangement for carrying out the method.
2. Description of the Related Art
In recent years, the requirements concerning increasingly narrower tolerances in relation to the thickness of rolled strips have continuously grown. It has soon been found that the influence of roll eccentricities has a disadvantageous effect when an attempt is made to maintain such narrow tolerances. For this reason, circuits have been developed which have the purpose of compensating the influence of roll eccentricities which reduce the quality of the rolled strip.
For example, it is known in the art initially to measure the eccentricity of the rolls when the rolls are moved together, to store the measured values and to reuse these measured values during the rolling procedure in order to compensate the roll eccentricities. Changes in the eccentricities of the rolls because of roll wear, thermally influenced changes, changes due to slippage etc., cannot be recognized by means of this compensation method. As a result, no compensation or only insufficient compensation takes place.
European patent 0170016 discloses a method for compensating the influence of roll eccentricities in which a disturbance signal is filtered out of the actual value of the rolling force, the roll adjustment and the stand springiness constant with the aid of the rate of rotation of the back-up rolls and in which the disturbance signal is reconstructed by means of oscillators. The reconstructed disturbance signal is used for controlling the thickness regulator of the roll stand.
The reasons for the eccentricities are grinding inaccuracies of the rolls, uneven wear, pressure variations in the bearings of the rolls, thermally caused eccentricities and others. All these irregularities can occur at each roll of a stand and are superimposed, so that very complex disturbance signal patterns result which can be reconstructed with reasonable accuracy only with substantially difficult and expensive oscillators. In addition, the rolls which are used have different diameters and are therefore driven with different rates of rotation, so that a filter controlled by the rate of rotation of the back-up rolls cannot be adjusted in an optimum manner for all rolls. Accordingly, the compensation of eccentricities according to this method is also insufficient.